How Computers Work – Course Information



E-Mail

Clients

▪ Microsoft Outlook

▪ Microsoft Outlook Express

▪ Eudora

▪ Thunderbird (Firefox)

▪ Web Mail

▪ AOL

▪ Yahoo

▪ Google (GMail)

▪ Windows Live (Hotmail)

▪ ISP Web Mail

▪ Roadrunner

▪ AT&T

The Real E-mail System

SMTP Server – Simple Mail Transfer Protocol (Outgoing Mail)

POP3 Server – Post Office Protocol (Incoming Mail)

IMAP Server – Internet Mail Access Protocol (Read Mail on the Server)

Instant Messaging

▪ Server allows clients to connect to it

▪ Server obtains IP Address and Port number for each client

▪ Server shares IP Address and Port numbers to clients trying to IM each other

▪ Clients begin communicating with each other without server assistance

Homework

Handouts - How Email Works

How Instant Messaging Works

Online - E-Mail & Instant Messaging Quiz

How E-mail Works

by Marshall Brain and Tim Crosby

E-mail Clients

You've probably already received several e-mail messages today. To look at them, you use some sort of e-mail client. Many people use well-known, stand-alone clients like Microsoft Outlook, Outlook Express, Eudora or Pegasus. People who subscribe to free e-mail services like Hotmail or Yahoo use an e-mail client that appears in a Web page. If you're an AOL customer, you use AOL's e-mail reader. No matter which type of client you're using, it generally does four things:

• Shows you a list of all of the messages in your mailbox by displaying the message headers. The header shows you who sent the mail, the subject of the mail and may also show the time and date of the message and the message size.

• Lets you select a message header and read the body of the e-mail message.

• Let's you create new messages and send them. You type in the e-mail address of the recipient and the subject for the message, and then type the body of the message.

• Lets you add attachments to messages you send and save the attachments from messages you receive.

The Real E-mail System

For the vast majority of people right now, the real e-mail system consists of two different servers running on a server machine. One is called the SMTP server, where SMTP stands for Simple Mail Transfer Protocol. The SMTP server handles outgoing mail. The other is either a POP3 server or an IMAP server, both of which handle incoming mail. POP stands for Post Office Protocol, and IMAP stands for Internet Mail Access Protocol. A typical e-mail server looks like this:

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The SMTP server listens on well-known port number 25, POP3 listens on port 110 and IMAP uses port 143.

The SMTP Server

Whenever you send a piece of e-mail, your e-mail client interacts with the SMTP server to handle the sending. The SMTP server on your host may have conversations with other SMTP servers to deliver the e-mail.

Let's assume that I want to send a piece of e-mail. My e-mail ID is brain, and I have my account on . I want to send e-mail to jsmith@. I am using a stand-alone e-mail client like Outlook Express.

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When I set up my account at howstuffworks, I told Outlook Express the name of the mail server -- mail.. When I compose a message and press the Send button, here's what happens:

1. Outlook Express connects to the SMTP server at mail. using port 25.

2. Outlook Express has a conversation with the SMTP server, telling the SMTP server the address of the sender and the address of the recipient, as well as the body of the message.

3. The SMTP server takes the "to" address (jsmith@) and breaks it into two parts: the recipient name (jsmith) and the domain name (). If the "to" address had been another user at , the SMTP server would simply hand the message to the POP3 server for (using a little program called the delivery agent). Since the recipient is at another domain, SMTP needs to communicate with that domain.

4. The SMTP server has a conversation with a Domain Name Server, or DNS. It says, "Can you give me the IP address of the SMTP server for ?" The DNS replies with the one or more IP addresses for the SMTP server(s) that Mindspring operates.

5. The SMTP server at connects with the SMTP server at Mindspring using port 25. It has the same simple text conversation that my e-mail client had with the SMTP server for HowStuffWorks, and gives the message to the Mindspring server. The Mindspring server recognizes that the domain name for jsmith is at Mindspring, so it hands the message to Mindspring's POP3 server, which puts the message in jsmith's mailbox.

If, for some reason, the SMTP server at HowStuffWorks cannot connect with the SMTP server at Mindspring, then the message goes into a queue. The SMTP server on most machines uses a program called sendmail to do the actual sending, so this queue is called the sendmail queue. Sendmail will periodically try to resend the messages in its queue. For example, it might retry every 15 minutes. After four hours, it will usually send you a piece of mail that tells you there is some sort of problem. After five days, most sendmail configurations give up and return the mail to you undelivered.

The POP3 Server

Your e-mail client connects to the POP3 server and issues a series of commands to bring copies of your e-mail messages to your local machine. Generally, it will then delete the messages from the server (unless you've told the e-mail client not to).

You can see that the POP3 server simply acts as an interface between the e-mail client and the text file containing your messages. And again, you can see that the POP3 server is extremely simple.

The IMAP Server

As you can see, the POP3 protocol is very simple. It allows you to have a collection of messages stored in a text file on the server. Your e-mail client (e.g. Outlook Express) can connect to your POP3 e-mail server and download the messages from the POP3 text file onto your PC. That is about all that you can do with POP3.

Many users want to do far more than that with their e-mail, and they want their e-mail to remain on the server. The main reason for keeping your e-mail on the server is to allow users to connect from a variety of machines. With POP3, once you download your e-mail it's stuck on the machine to which you downloaded it. If you want to read your e-mail both on your desktop machine and your laptop (depending on whether you're working in the office or on the road), POP3 makes life difficult.

IMAP (Internet Mail Access Protocol) is a more advanced protocol that solves these problems. With IMAP, your mail stays on the e-mail server. You can organize your mail into folders, and all the folders live on the server as well. When you search your e-mail, the search occurs on the server machine, rather than on your machine. This approach makes it extremely easy for you to access your e-mail from any machine, and regardless of which machine you use, you have access to all of your mail in all of your folders.

How Instant Messaging Works

by Jeff Tyson and Alison Cooper

Introduction to How Instant Messaging Works

The Internet has revolutionized the way we communicate. E-mail has been the most rapidly adopted form of communication ever known. Less than two decades ago, not many people had heard of it. Now, many of us e-mail instead of writing letters or even calling people on the phone. People around the world send out billions of e-mail messages every day.

But sometimes even e-mail isn't fast enough. You might not know if a person you want to e-mail is online at that moment. Also, if you're e-mailing back and forth with someone, you usually have to click through a few steps. This is why instant messaging (IM) has become so popular.

With IM, you can keep a list of people you interact with. You can IM with anyone on your buddy list or contact list as long as that person is online. You type messages to each other into a small window that shows up on both of your screens.

Most IM programs provide these features:

• Instant messages - Send notes back and forth with a friend who is online

• Chat - Create a chat room with friends or co-workers

• Web links - Share links to your favorite Web sites

• Video - Send and view videos, and chat face to face with friends

• Images - Look at an image stored on your friend's computer

• Sounds - Play sounds for your friends

• Files - Share files by sending them directly to your friends

• Talk - Use the Internet instead of a phone to actually talk with friends

• Streaming content - Real-time or near-real-time stock quotes and news

• Mobile capabilities - Send instant messages from your cell phone

History of Instant Messaging

Before the Internet became popular, a lot of people were already online. The major online services, such as America Online (AOL), Prodigy and CompuServe, were the main way that ordinary people could connect and communicate with each other online. Online services provide the actual interface that you use when you're connected to the service, which creates a targeted experience for users.

In the early 1990s, people began to spend more and more time on the Internet. Creative software developers designed chat-room software and set up chat rooms on Web servers. In a chat room, a group of people can type in messages that are seen by everyone in the "room." Instant messages are basically a chat room for just two people.

Instant messaging really exploded on the Internet scene in November 1996. That's when Mirabilis introduced ICQ, a free instant-messaging utility that anyone could use. ICQ, shorthand for "I seek you," uses a software application, called a client, that resides on your computer. The client communicates with an ICQ server whenever you are online and the client is running.

In 1997, AOL, considered the pioneer of the online community, gave its users the ability to talk in real time with each other through chat rooms and instant messages. In June 1998, AOL acquired Mirabilis and ICQ.

The ICQ model is the basis for most instant-messaging utilities on the market today. In the next section we'll examine how these services work.

Using Instant Messaging

Look at the steps below to understand exactly what happens with an instant-messaging service.

1. You go to the download page and get a copy of the free software client for your computer.

2. You install the software and open the client.

3. The client tries to connect to the server. It uses a proprietary protocol for communication.

4. Once the client is connected to the server, you can enter your name and password to log in to the server. If this is your first time on, you can sign up for an account and immediately begin using it. When the server verifies your name and password, you are logged in.

5. The client sends the server the connection information (IP address and number of the port assigned to the client) of the computer you are using. It also provides the user with the names of everyone in your contacts list.

6. The server creates a temporary file that has the connection information for you and the list of your contacts. It then checks to see if any of the users in your contact list are currently logged in.

7. If the server finds any of your contacts logged in, it sends a message back to the client on your computer with the connection information for that user. The server also sends your connection information to the people in your contact list that are signed on.

8. When your client gets the connection information for a person in your contact list, it changes the status of that person to "online." You click on the name of a person in your contact list who is online, and a window opens that you can enter text into. You enter a message and click "send" or hit return to communicate with that person.

9. Because your client has the IP address and port number for the computer of the person that you sent the message to, your message is sent directly to the client on that person's computer. In other words, the server is not involved at this point. All communication is directly between the two clients.

10. The other person gets your instant message and responds. The window that each of you sees on your respective computers expands to include a scrolling dialog of the conversation. Each person's instant messages appears in this window on both computers.

11. When the conversation is complete, you close the message window. Eventually, you go offline and exit. When this happens, your client sends a message to the server to terminate the session. The server sends a message to the client of each person on your contact list who is currently online to indicate that you have logged off. Finally, the server deletes the temporary file that contained the connection information for your client. In the clients of your contacts that are online, your name moves to the offline status section.

Instant Messaging Providers

Not long after AOL bought ICQ, AOL Instant Messenger (AIM) became the IM leader. In the past few years, though, a number of services have cut into AIM's audience. Windows Live Messenger (formerly MSN Messenger) and Yahoo! Messenger, in particular, have become widely used around the globe. Google recently introduced its IM system, Google Talk. Proprietary multiprotocol applications like Trillian and Pidgin, which allow users to IM on several services at once, are also rapidly gaining in popularity. According to a 2006 study by comScore, which measures Internet usage, about half of the online population uses IM.

The major IM utilities use a proprietary protocol that is not understood by other instant-messaging services, so users of one service are usually blocked from contacting members of another. A key reason for AIM's initial popularity is that it allowed AOL subscribers to communicate with nonmembers. One of the provisions of the AOL-Time Warner merger in 2001 was that AOL would allow access to the AOL community and AIM protocol, but since then other instant-messaging utilities have provided more accessibility. Microsoft and Yahoo! joined forces in 2006 to allow their users to communicate with each other on one service.

It is important to note that instant messaging is not considered a secure way to communicate. Messages and connection information are maintained on servers controlled by the provider of your IM utility. Most utilities do provide a certain level of encryption, but they are not so secure that you should send any confidential information through the system. There have been reported cases of IM user logs being captured and used by nefarious sorts, and hackers have been known to instant-message virus-infected files. Voice Over Internet Protocol is thought to be more vulnerable to infiltration than text-based instant-messaging.

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